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We have drain boot connected to the gas headers upstream the dehydration plant. The liquid from the drain boot is sent to the Blow down system while the vapor is recycled back to the gas headers.

The pressure on the gas headers is around 400 psig and the temperature is between 100 to 120 F. The pressure of the blow down is 2 psig. The liquid level in the drain boot is controlled by level transmitter which sends signal to the level control valve to open or closed.

We are experiencing a problem that the ice is formed on the line downstream the control valve and the level increased at the same time while the control valve opens fully. However, after a while the level decreases and the ice melts and the level control valve start to control.

The valve is two phase globe valve, Is it really hudration???

[Art Montemayor]

What is the liquid you are draining from the boot? Is it water – or does it have the properties similar to water? If the expansion of the liquid is causing a plug-up in the drain line, I doubt that it is a hydrocarbon. But I could be wrong. You don’t tell us the temperature downstream of the level blow down. Here, I am presuming you have rapid cooling due to expansion evaporation – as often happens with a liquid hydrocarbon. This is a normal refrigerating effect.

You also fail to tell us what you call or identify as ice being formed. Is the “ice” formed on the outside – or internally, in the drain line? I fail to see how you could know that ice is being formed inside the drain line. You certainly can’t see it being formed.

I suspect and speculate that what is happening is that you have liquid hydrocarbons (butanes, propanes, etc.) being expanded by the level control valves and these contain a level of water content. Upon expansion and subsequent cooling, the drained liquids form a hydrocarbon hydrate (a solid gel) that plugs the drain line. You are seeing ice formed on the exterior of the drain line. This is the solidification of the external humidity that condenses and solidifies on the external surface of the pipe. When the drain pipe warms up (and the external water ice melts), the internal hydrate gel dissolves and the drain line is freed up. If my guess is right, then the way to keep your drain line free is to inject a hydrate inhibitor into it – like methanol. This is what is conventionally done out in the field.